The invention relates to a process for producing grains of blasting agents consisting of rustproof cast stainless steel, in which a granulate is produced first from a the melt of an iron-chromium-carbon alloy capable of being hardened. Said alloy is then passed through a heat treatment at  greater than 900xc2x0 C. for hardening it, and subsequently crushed to grains with sharp edges.
For the blasting treatment of workpieces consisting of stainless materials it is necessary to employ stainless blasting agents as well because rusting blasting agents such as steel shot or steel gravel leave iron-containing residues on the surface of the workpiece. Due to the oxidation of adhering iron residues, undesirable stains of rust then appear within a very short time. In addition to nonmetallic, mostly mineral blasting agents such as, for example electro-corundum, silicon carbide or glass, stainless metallic blasting agents are known as well. To be mentioned in this connection is stain steel blasting shot consisting of corrosion-resisting steel alloys. Such material offers a number of advantages vis-à-vis the mineral blasting agents. For example, a substantially prolonged useful life can be achieved with the metallic blasting grains on the usual blasting equipment because the stainless steel, by virtue of its greater ductility, is crushed in the course of the blasting treatment to a substantially lesser degree. Stainless steel blasting agents have been successfully employed especially when used in blasting plants equipped with centrifugal wheels because of their good wear properties conditioned by its high impact strength.
Two categories of blasting agents consisting of non-corrosive cast stain less are known. These are granulates consisting of spherical grains that are made of steel materials with medium hardness ( less than 45 HRC), on the one hand. As disclosed in JP 61 257 775, on the other hand, grains having sharp edges and consisting of hardened cast chromium iron ( greater than 60 HRC) are employed because enhanced abrasion properties can be achieved with such grains.
Substantially higher manufacturing expenditure and additional process steps are required in the manufacture of the hardened granulate with sharp edges vis-à-vis the blasting agent grains of the first category. In the manufacturing process according to JP 61 257 775, a granulate consisting of substantially round grains is produced first based on a melt of a chromium-cast iron alloy that is capable of hardening. Said granulate is hardened by quenching it at 1000xc2x0 C. to 1100xc2x0 C. in water, following a thermal treatment. The grains are subsequently crushed, so that a material with sharp edges is obtained.
The drawback of said method is that the undesirable oxidation of the material is favored by quenching the hot steel in water, said steel having a temperature of more than 1000xc2x0 C. Furthermore, the achievable cooling rate is highly limited when water is used (steam phase). However, effective quenching is absolutely required in order to obtain a material that is as brittle as possible. This is the precondition that has to be satisfied so that the grains can be crushed later in such a way that the desired granulate with sharp edges is produced.
The present invention is accordingly based on the problem of providing a process for producing corrosion-proof blasting agents, in which oxidation of the granulate can be excluded during and after the final heat treatment, and in which the brittleness of the material achieved in the hardening step is adequately high for crushing the blasting grains with simple means to obtain a granulate with sharp edges.
Said problem is solved in conjunction with a production process of the type specified above in that the thermal treatment is carried out at  greater than 900xc2x0 Celsius in a reducing atmosphere, and in that a reducing gas or gas mixture is used in the subsequent cooling step as well.
Owing to the fact that the granulate is exclusively subjected to a reducing atmosphere in the hardening phase, the advantage ensues that undesirable oxidation of the material is reliably avoided.
The reducing atmosphere usefully is a gas mixture that contains hydrogen and nitrogen. It has been found in practical life that a gas mixture containing from 60% to 80% hydrogen and from 20% to 40% nitrogen is particularly suited for the process as defined by the invention. The best results were achieved with 70% hydrogen and 30% nitrogen.
Special process steps have to be maintained in order to produce blasting agents from a cast iron-chromium alloy. The use of an iron-chromium-carbon alloy with at least 2% carbon and at least 30% chromium results in a material that can be hardened in a corrosion-resistant manner, whereby degrees of hardening of  greater than 60 HRC can be achieved without problems. A material is obtained in this way that is characterized by high resistance to oxidation and excellent resistance to wear. The use of the alloy specified above is therefore especially useful in the process as defined by the invention because it offers the combination of a material that can be hardened in a good way and is resistant to corrosion at the same time.
For crushing the hardened granulate it is useful if a pulsed mill is employed. A tubular oscillating mill is particularly suited for producing from the hardened starting material the desired granulate with sharp edges.
If the blasting agent is employed for the surface treatment of metallic workpieces it is useful if it is present classified according to grain size. The production process as defined by the invention can be followed for such a treatment by an additional process step downstream for fractionating the grains. The adjustment of the desired grain mixture is achieved with an extra process step.